The present invention relates to rotating vacuum fingers for removal or peeling off of print media, typically paper, from an impression drum, and, more particularly, but not exclusively to the removal of print media from a drum that prints multiple colors in multiple rotations or separations.
Vacuum fingers are hollow tubes that carry a vacuum, and have suckers at one end to transfer the suction to paper or any other print medium that it is desired to move, feed or remove within a printing, photocopying or any other environment in which paper feeding is necessary. The present disclosure relates specifically to rotary mounted vacuum fingers that are intended to counter-rotate in association with an impression drum, and remove the paper or other print medium from the impression drum following the application of the printing impression to the paper. In such a case the paper wraps onto the drum to receive the impression and then meets the vacuum finger which peels the paper from the drum.
Patents relating to rotary systems that utilize suckers to assist with feeding of paper in association with a drum include U.S. Pat. Nos. 5,431,384, 6,313,859, 6,639,622, 6,048,120, GB 735,477 and U.S. Pat. No. 6,120,143.
In a typical drum the paper rotates about the drum once, receives an impression with a single color ink and is then peeled away. However, in a recent development, multi-color print drums have become available in which several colors are applied to the paper in a spread of several rotations or separations, one color being applied per separation. Such a development has been enabled by inks that can easily be cleaned. In such a case, the paper meets the suction fingers at each separation, but suction is only applied at the end of the spread. In some cases up to seven colors may be applied in a spread, requiring seven separations. Suction is only applied following the last rotation.
Reference is now made to FIG. 1 which shows part of a printed sheet and illustrates zones 10 of colored printing. The zones comprise a non-printed region 12, and a printed region 14. Within the printed region 14 are a series of marks 16 which spoil the finish of the printed image.
The present inventors attempted to determine how the marks were formed. In doing so they studied the operation of the peeling process for removing paper from the impression drum. The peeling process is illustrated in FIG. 2 which is a simplified schematic diagram showing a cross section of an impression drum with paper and associated paper peeling apparatus. In FIG. 2, an impression drum 20 is fed with paper 22 on which an impression is applied. The paper 22 is then peeled from the impression drum by the paper peeling apparatus. In the paper peeling apparatus, a vacuum is applied to the paper 22 by a suction cap 24. The suction cap 24 is fed with the vacuum via a vacuum finger 26 which is a hollow tube connected to a central vacuum source.
In use the drum 20 rotates and the paper rotates on the drum. A plurality of vacuum fingers 26 rotate, in the opposite sense to drum 20, about drive shaft 28, such that the vacuum fingers touch the edge of the drum tangentially at one point in their rotation, the point at which the paper is to be peeled from the drum. At the point of peeling, the rotational velocities at the center of the suction cap and of the paper are matched so as to minimize any impact damage of the suction cap on the paper and allow the vacuum fingers to peel the paper smoothly from the drum.
The present inventors studied the above mechanism for possible causes of the impact damage and it was noted that the suction caps have a finite radius and that the rotational velocities between the suction cap and the paper are only matched for a certain point on that radius. That is to say the velocity can be matched for the center of the suction cap in which case there is a speed mismatch with the brim 30 of the cap, or the speed can be matched with the brim 30, in which case there is a mismatch with the center. Either way there is a possible cause for impact damage. Furthermore the brim 30 of the suction cap extends outwardly beyond the circumference described by the locus of travel of the center of the suction cap, meaning that the brim presumably impacts and attempts to dig in to the paper at its point of initial contact.
The impacts are presumably made worse in the case of multi-color printing in a spread over multiple separations. As explained above, the vacuum is switched off when additional rotations of the paper are required for remaining colors, however the fingers continue to rotate and impacts still occur even during rotations when the paper is not peeled from the drum, simply making the impact damage more noticeable.
A number of prior art patents describe uses of suckers in ways that do not impact the medium being fed, but none of these are suitable for paper pick up from a rotary drum. U.S. Pat. No. 5,997,458 describes a rotary object feeder in which suckers are used to transfer cardboard objects. The suckers are mounted on a planetary device and thus are not suitable for paper feed from a drum. In the same context U.S. Pat. No. 5,511,722 teaches an offset rotary mechanism for transferring cartons from a stack to a belt. Again the system is not intended for paper feeding from a rotating drum.
Finally U.S. Pat. No. 5,431,384 teaches a pickup and transfer roller in which the entire roller is lifted during part of the transfer cycle.
The present inventors recognized a need for a paper peeling system for removing paper from a drum, which is devoid of the above limitations.